1. Field of the Invention
This invention concerns II-IV-type semiconductor crystals having nanometer dimensions and a process for their production.
2. Background Art
The optical spectra of semiconductor crystallites whose dimension is comparable to the bulk exciton diameter show quantum confinement effects. Henglein, A., Top. Curr. Chem. (1988) 143:113-180; Brus, L. E., J. Phys. Chem. (1986) 90:2555; Brus, L. E., IEEE J. Quantum Electron (1987) QE-22:1909; Shiang, J. J. et al., J. Chem. Phys. (1990) 92:3232; Alivisatos, A. P. et al., J. Chem Phys. (1988) 89:4001; Alivisatos, A. P. et al., J. Chem. Phys. (1989) 90:3463; Alivisatos, A. P. et al., J. Chem. Phys. (1988) 89:5979. To date, experimental studies of nanometer-size crystallites have been restricted to II-VI and I-VII semiconductors, while III-V semiconductors, including one of the most important direct band gap semiconductors, GaAs, have not yet been studied in this form because of the numerous difficulties encountered in their preparation., Compared to the I-VII and II-VI semiconductors, the III-Vs have a greater degree of covalent bonding, a less ionic lattice, and larger exciton diameters (the exciton diameter in GaAs is 190 .ANG., compared to 60 .ANG. for CdS). For this reason, quantum size effect on the optical spectra have been . predicted to be more pronounced in the III-V class of materials than in the II-VIs, Brus, L. E., J. Chem. Phys. (1983) 79:5566, and crystallites of GaAs are more likely to find application in optical devices than CdS clusters. Schmitt-Rink, S. et al., Phys. Rev. B (1987) 5:8113; (9) Miller, D.A.B. et al., Phys. Rev. B (1986) 33:6976; Appl. Phys. Lett. (1988) 52:2154.
Thus, there is an unfilled need for relatively monodisperse, redissolvable, crystalline, nanometer-size III-V semiconductor particles. Prior methods of producing GaAs have not yielded such products. For example, surface-bound GaAs nanocrystals with broad size distribution have been prepared by molecular beam epitaxy on amorphous silica. Sandroff, C. et al., Science (1989) 245:391. Byrne, E. K. et al., Science (1988) 241:332, reported a liquid-phase reaction which they said gave gallium arsenide particles, without supporting evidence.
Another preparation method has been recently described by Wells and co-workers. Wells, R. L. et al., Chem. Mater. (1989) 1:4; Wells, R. L. et al., Mater. Res. Soc. Symp. Proc. (1989) 131:45. The Wells et al. preparation route is based on the finding that GaCl.sub.3 and As(SiMe.sub.3).sub.3 react in low-boiling hydrocarbons to give solid products, which on heating afford bulk polycrystalline GaAs. Our X-ray diffraction experiments on the black GaAs powder prepared in this manner show that the domain size is 100 .ANG..